A plastic substrate is an indispensable part of a flexible electronic display device. Currently, there are two main methods for fabricating a thin film of plastic substrate, wherein one method is solvent casting, while another method is melt extrusion. Selection of the methods to form a film is in accordance with the difficulty of the melt process of the thermoplastic polymer. Typically, the melt process for forming a film is suitable for a polymer with its glass transition temperature (Tg) lower than 140° C., such as polyethylene terephthalate (PET, Tg≈80° C., Tm≈250° C.) which is a semi-crystalline polyester and can be melt-processed easily. However, for a polymer with a glass transition temperature higher than 140° C., such as polycarbonate (PC, Tg≈150° C.) and polyethersulfone (PES, Tg≈220° C.), the segmental motion of the polymer chain units become difficult, and a high melt temperature is required for thermal plasticizing. If the melt process leads to a certain degree of polymer degradation and a lowering of optical transparency, the solvent casting method is more likely to be chosen to form a film with a desirable optical transparency. For a polymer with a very high glass transition temperature, such as polyimide (PI, Tg>300° C.), the melt extrusion method for film forming is not feasible and the solvent casting method is used.
When a transparent film of plastic substrate is formed by a solvent casting method, the roughness of the film is usually reduced compared to that being formed by the melt extrusion method. However, the cost of the solvent casting method is higher and the mechanic property of the resulting film is poorer. Moreover, the organic solvents used in the solvent casting method often lead to environment pollution. The melt extrusion method, not only provides high productivity, but the mechanical property and the surface smoothness of the film can be modified via a biaxial stretching process. Further, the problem of the environment pollution caused by the solvent casting method is precluded. Since the cost of melt extrusion is lower than that of solvent casting, melt extrusion is often applied when cost and productivity are being considered.
Applying a plastic film, such as a polyester film, as a value-added optical film, or a film material in a flexible display device or a solar cell, is actively pursued by the polyester industry.